Description: (Applicant's abstract) Virion structures will be determined for adenovirus and the adenovirus-like bacteriophage PRD1 in atomic detail and correlated with viral functions. Both virions are large with a relatively complicated, but similar, icosahedral architecture including specialized vertex structures responsible for recognition and entry. The experimental approach combines X-ray crystal structures of the coat proteins with cryo-electron microscopy (EM) image reconstructions of virions to obtain initial models. The molecular positions are then refined by computational methods to define intermolecular interactions and reveal other virion components. Adenovirus causes various human diseases and is an important vector for human gene therapy. The crystal structure of hexon, the major coat protein, was recently determined at 2.5A resolution for type 5 (ad5). The model of the 951-residue chain is better than an earlier 2.3A model for the 967-residue ad2 hexon. Although the overall fold is maintained, sequence assignments in two regions differ significantly. The new refinement methods used for ad5 will be applied to ad2, and structures for the more distantly related ad12 and avian hexons determined. These structures will be compared to find the basis for hexon's extraordinary molecular stability, and to show how different serotypes have evolved. The final hexon model will facilitate engineering to produce virions with modified outer hexon surfaces as immunologically distinct variants for gene delivery. The computational model of the 240-hexon capsid will define the binding sites for minor "cementing" proteins that play a key role in stabilizing the virion and suggest how analogs could be designed to disrupt infection. PRD1 is an unusual membrane-containing dsDNA bacteriophage. The recently-determined crystal structure of its 394-residue major coat protein, P3, has revealed an unexpected evolutionary relationship between PRD1 and adenovirus. A rigid-body model of P3 will be fitted to EM images of virions, empty capsids, and P3 shells to improve the current capsid model and reveal how P3 interacts with the internal membrane. Two other PRD1 proteins have been crystallized and their structures will be determined. The 64 kDa monomeric vertex protein, P2, is responsible for attachment and so is analogous to the receptor-binding adenovirus fiber. A structure for the 38 kDa tetrameric assembly factor, P17, that is essential for virion formation, will shed light on proteins that are poorly understood despite their critical and possibly general role in viral assembly.